In wafer-to-wafer bonding technology, various methods have been developed to bond two package components (such as wafers) together. The available bonding methods include fusion bonding, eutectic bonding, direct metal bonding, hybrid bonding, and the like. In the fusion bonding, an oxide surface of a wafer is bonded to an oxide surface or a silicon surface of another wafer. In the eutectic bonding, two eutectic materials are placed together, and are applied with a high pressure and a high temperature. The eutectic materials are hence melted. When the melted eutectic materials are solidified, the wafers are bonded together. In the direct metal-to-metal bonding, two metal pads are pressed against each other at an elevated temperature, and the inter-diffusion of the metal pads causes the bonding of the metal pads. In the hybrid bonding, the metal pads of two wafers are bonded to each other through direct metal-to-metal bonding, and an oxide surface of one of the two wafers is bonded to an oxide surface or a silicon surface of the other wafer.
The previously developed bonding methods have their advantageous features and disadvantageous features. For example, the fusion bonding requires low forces, and may be performed at room temperature. However, since there is no electrical connection between the bonded wafers, extra electrical connections need to be made to interconnect the bonded wafers. The Eutectic bonding does not require high-quality surfaces, and hence the pre-requirement for a successful bonding is loosened. The accuracy of the eutectic bonding, however, is low, and there may be metal-squeeze problem during the eutectic bonding due to the melting of the bonding metals. The direct metal-to-metal bonding has a high accuracy. The throughput, however, is very low. In the hybrid bonding, there may be dielectric delamination problem. The reason is that in the hybrid bonding, metal pads have higher Coefficient of Thermal Expansion (CTE) than the dielectric layers and silicon at the surfaces of the bonded wafers. The greater expansion of the metal pads causes the oxide of one wafer to be pushed away from the silicon or the oxide on the other wafer.